The present invention relates to a separate plate for use in a valve body of an automatic transmission. The presetn invention also pertains to a process for producing a separate plate of the type described above.
FIG. 4 shows a general structure of a valve body of a typical automatic transmission. More specifically, the valve body comprises a lower valve body member 1 and upper valve body members 2 and 3. Inside the valve body defined by these valve body members are formed grooves for accommodating various kinds of valve, for example, a manual valve 4 and an oil cooler bypass valve 5, and grooves for accommodating accumulators, together with oil passages. A separate plate is sandwiched between the lower valve body member 1 and the upper valve body members 2, 3 and is fastened thereto by means of bolts. The separate plate has a first gasket sheet 6, a plate 7 for hydraulic control and a second gasket sheet 8.
FIG. 5 is a plan view of the lower valve body member 1, while FIG. 6 is a plan view of the upper valve body members 2 and 3.
Referring to FIG. 7, which is a plan view of the plate 7, the plate 7 is provided with a multiplicity of bores which provide communication between the lower valve body member 1 and the upper valve body members 2, 3 to control the oil-hydraulic circuit. The respective diameters of these bores are designed so that predetermined levels of pressure are maintained in the oilhydraulic circuit.
FIG. 8 is a plan view of one of the gaskets 6 and 8. The gasket is, as a matter of course, provided with bores in correspondence to the multiplicity of bores in the plate 7, but the diameters and configurations of these bores are not necessarily the same as those of the bores in the plate 7. As will be understood by seeing through FIGS. 7 and 8 laid one upon the other, the bores b, for example, in the gasket which correspond to the orifices a in the plate 7 have larger diameters than those of the latter. This is because the diameters of the orifices are required to have a relatively high degree of accuracy and it is therefore necessary to prevent the orifices from being partially closed by the gasket due to misalignment between the plate 7 and the gasket. Further, some orifices are chamfered for the purpose of stabilizing the flow velocity of oil and it is therefore necessary in order to avoid an interference with this function to set the diameters of the bores in the gasket so as to be larger than the diameters of the corresponding orifices in the plate 7. In addition, it is necessary to provide the gasket with openings c which allow the springs of accumulators or the balls of ball check valves to be in contact with the plate 7.
However, the above-described prior art (hereinafter referred to as "the prior art .circle.1 "), in which the plate 7 is clamped at both sides thereof between the gaskets 6 and 8 and rigidly secured by menas of bolts, has the problem that it is necessary to confirm the obverse and reverse sides of the gaskets and align these members when rigidly secured by means of bolts. The prior art .circle.1 also suffers from inferior assembling efficiency, that is, there is a fear of the gaskets being folded during assembly.
Further, the prior art .circle.1 has the problem that the gasket may be cracked as shown in FIG. 9 due to the difference in thermal expansion coefficient between the gasket and the valve body in actual use where high-temperature and high-pressure oil acts thereon. When the bolt fastening is relatively strong, cracks are generated in the end face portions of the gasket where it is in contact with the casing or the valve body, whereas, when the bolt fastening is relatively weak, cracks start from bores in the gasket, such as bolt receiving bores and oil bores. If cracks are generated in the gasket, the sealing properties deteriorate, so that oil may leak out to lower the level of oil pressure, thus causing a mal-operation of the oil-hydraulic controller. There is also a fear of fragments of the gasket entering the oil-hydraulic circuit to cause sticking of valves, which leads to a failure.
To solve the above-described problems, a method wherein a rubber material or the like is applied to each side of a plate by means of coating or bonding and then this plate and two gaskets are bonded together in one unit is proposed, for example, in Japanese Patent Public Disclosure No. 61-109960 (1986). This prior art (hereinafter referred to as "the prior art .circle.1 ") suffers, however, from a low degree of punching accuracy since bores, such as orifices, are formed in the plate by means of punching after both sides of the plate have been coated with a rubber material or the like. In the prior art .circle.2 also, each gasket must be provided with bores b in correspondence to the orifices in the plate, the bores b having larger diameters than those of the corresponding orifices, and some of the orifices need to be chamfered. Further, the gasket must be provided with openings which allow the springs of accumulators or the balls of ball check valves to be in contact with the plate 7. These steps involve complicated and troublesome operations, as described above.